Soft solder is a fusible alloy, typically consisting of tin and lead, which is used for the purpose of joining together two or more metals at temperatures below their melting points. In addition to tin and lead, solders may occasionally contain varying amounts of other materials, such as antimony, bismuth, cadmium, or silver, which are typically added for the purpose of varying the physical properties of the alloy. However, in many solders, some of these elements, notably antimony, are only present as impurities. Solder is widely used in the electronics industry for attaching electrical components to printed circuit boards. Printed circuit boards typically have an electrical conductor pattern consisting of a thin metal sheet, etched to form the pattern. In order to successfully attach the electrical components to the printed circuit board, there must be a metallurgical affinity between the two metals that are going to be soldered together, the metals should be free from contamination, there must be complete and adequate metallic contact between the solder and the metals that are to be soldered, and there must be a temperature adequate for sufficient alloying of the metal. The solder provides the attachment by virtue of an intermetallic solution, which takes place at the soldering temperature.
One technique used to assemble electronic assemblies is to screen print a solder paste, (consisting of flux, vehicle, and alloy), onto the board, place the component in the wet solder paste, and then reflow the solder paste to join the component to the PCB. However, screen printing lacks the flexibility required in today's build-to-order assembly factories, and results in solder voids after reflow, degrading the solder joint strength.
Another method of assembling electronic assemblies is by a process known as solder cladding or solid solder deposition. This is done by screen printing and reflowing a solder paste on the solderable surfaces of a printed circuit board (PCB) during the PCB fabrication. The advantage of this method is that the electronic components can then be placed directly on the PCB, without having to print solder paste. One of the disadvantages of this method is that the solder that is clad onto the PCB results in a highly domed surface on the solder pads that degrades the accuracy of component placement. The parts and flux tend to slide off of the domed surface when they are placed, and while the circuit board is subjected to accelerations moving down the assembly line. Some have attempted to solve this problem by reducing the amount of solder clad onto the solder pad, thus decreasing the curvature of the domed surface. However, reducing the volume of solder decreases the total wetting forces of the solder, reducing the desirable tendency of the components to self-center during the assembly reflow operation. This self-centering tendency is extremely important to assure accurate alignment and high-yield soldering. In addition, it is desirable to maximize the amount of solder deposited on the board during the cladding process in order to create a full solder fillet between the component and the PCB. Still others have attempted to eliminate the domed pads by flattening the pad after the solder is deposited, as in the OPTIMASK.RTM. method developed by DuPont and the SIPAD process developed by Siemans AG. In both cases, additional steps of flattening the domed pad are required.
Clearly, it would be a benefit to the electronics industry if a method of soldering could be devised that includes the advantages of both solder cladding and solder printing, but avoids the disadvantages of both processes.